[[!meta copyright="Copyright © 2010 Free Software Foundation, Inc."]] [[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable id="license" text="Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled [[GNU Free Documentation License|/fdl]]."]]"""]] \#hurd, freenode, 2010 humm... why does tmpfs try to use the default pager? that's a bad idea, and probably will never work correctly... * slpz is thinking about old issues tmpfs should create its own pagers, just like ext2fs, storeio... slopez@slp-hurd:~$ settrans -a tmp /hurd/tmpfs 10M slopez@slp-hurd:~$ echo "foo" > tmp/bar slopez@slp-hurd:~$ cat tmp/bar foo slopez@slp-hurd:~$ :-) slpz: woo you fixed it? pochu: well, it's WIP, but reading/writing works... I've replaced the use of default pager for the standard pager creation mechanism slpz: err... how is it supposed to use swap space if not using the default pager? slpz: or do you mean that it should act as a proxy, just allocating anonymous memory (backed by the default pager) itself? antrik: the kernel uses the default pager if the application pager isn't responsive enough antrik: it will just create memory objects and provide zerofilled pages when requested by the kernel (after a page fault) youpi: that makes sense I guess... but how is that relevant to the question at hand?... antrik: memory objects will contain the data by themselves antrik: as youpi said, when memory is scarce, GNU Mach will start paging out data from memory objects to the default pager antrik: that's the way in which pages will get into swap space (if needed) the thing being that the tmpfs pager has a chance to select pages he doesn't care any more about slpz: well, the point is that instead of writing the pages to a backing store, tmpfs will just keep them in anonymous memory, and let the default pager write them out when there is pressure, right? youpi: no idea what you are talking about. apparently I still don't really understand this stuff :-( ah, but tmpfs doesn't have pages he doesn't care about, does it? antrik: yes, but the term "anonymous memory" could be a bit confusing. antrik: in GNU Mach, anonymous memory is backed by a memory object without a pager. In tmpfs, nodes will be allocated in memory objects, and the pager for those memory objects will be tmpfs itself slpz: hm... I thought anynymous memory is backed by memory objects created from the default pager? yes, I understand that tmpfs is supposed to be the pager for the objects it provides. they are obviously not anonymoust -- they have inodes in the tmpfs name space but my understanding so far was that when Mach returns pages to the pager, they end up in anonymous memory allocated to the pager process; and then this pager is responsible for writing them back to the actual backing store am I totally off there?... (i.e. in my understanding the returned pages do not reside in the actual memory object the pager provides, but in an anonymous memory object) antrik: you're right. The trick here is, when does Mach return the pages? antrik: if we set the attribute "can_persist" in a memory object, Mach will keep it until object cache is full or memory is scarce or we change the attributes so it can no longer persist, of course without a backing store, if Mach starts sending us pages to be written, we're in trouble so we must do something about it. One option, could be creating another pager and copying the contents between objects. another pager? not sure what you mean BTW, you didn't really say why we can't use the default pager for tmpfs objects :-) well, there're two problems when using the default pager as backing store for translators 1) Mach relies on it to do swapping tasks, so meddling with it is not a good idea 2) There're problems with seqnos when trying to work with the default pager from tasks other the kernel itself (probably, the latter could be fixed) antrik: pager's terminology is a bit confusing. One can also say creating another memory object (though the function in libpager is "pager_create") not sure why "meddling" with it would be a problem... and yeah, I was vaguely aware that there is some seqno problem with tmpfs... though so far I didn't really understand what it was about :-) makes sense now anyways, AIUI now you are trying to come up with a mechanism where the default pager is not used for tmpfs objects directly, but without making it inefficient? slpz: still don't understand what you mean by creating another memory object/pager... (and yeat, the terminology is pretty mixed up even in Mach itself) antrik: I meant creating another pager, in terms of calling again to libpager's pager_create slpz: well, I understand what "create another pager" means... I just don't understand what this other pager would be, when you would create it, and what for... antrik: oh, ok, sorry antrik: creating another pager it's just a trick to avoid losing information when Mach's objects cache is full, and it decides to purge one of our objects anyway, IMHO object caching mechanism is obsolete and should be replaced I'm writting a comment to bug #28730 which says something about this antrik: just one more thing :-) if you look at the code, for most time of their lives, anonymous memory objects don't have a pager not even the default one only the pageout thread, when the system is running really low on memory, gives them a reference to the default pager by calling vm_object_pager_create this is not really important, but worth noting ;-)